When an energy-storing phosphor (e.g., stimulable phosphor, which gives off stimulated emission) is exposed to radiation such as X-rays, the phosphor absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when it is exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the panel; sequentially scanning the panel with stimulating light such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support and a phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support can be omitted. Further, a protective layer is normally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical damage.
Various kinds of energy-storing phosphor layers are known. For example, the phosphor layer can comprise a binder and energy-storing phosphor particles dispersed therein. Alternatively, the phosphor layer can comprise agglomerate of an energy-storing phosphor without binder. The binderless phosphor layer can be formed by a gas phase-accumulation method or by a firing method. For example, in the gas phase-accumulation method, the phosphor or material thereof is vaporized (or sputtered) and accumulated on a substrate to form a layer of the phosphor in the form of columnar crystals. Thus prepared phosphor layer consists of only the phosphor, and there are cracks among the columnar crystals of phosphor. Accordingly, the stimulating light can be applied efficiently enough and the emission can be collected also efficiently enough to improve the sensitivity. In addition, since the stimulating light is kept from scattering horizontally, a radiation image of high sharpness can be obtained.
JP-A-2001-74898 discloses a process for read-out of radiation image and an apparatus for the process in the disclosed process, a photo-stimulable phosphor screen comprising divalent europium activated cesium halide (chloride or bromide) phosphor is used. The screen has a surface area not larger than Smax. After a radiation image beforehand recorded in the screen is read out, the screen is irradiated with an erasing light in the wavelength region of 300 to 1,500 nm so as to erase the radiation energy remaining in the screen. The erasing light is emitted from a light source composition having an electric power not larger than Smax×1 J. Examples in the publication teach that the screen comprising CsBr:Eu2+ phosphor has better erasing characteristics than the conventional BaFBr:Eu2+ phosphor-containing screen and, accordingly that the radiation energy remaining therein can be erased well with an erasing light emitted from a source of relatively small power.
As described above, a radiation image storage panel is repeatedly used in the radiation image recording and reproducing method. The erasing step (in which the whole surface of the storage panel is irradiated with an erasing light) is, therefore, generally carried out after the read-out procedure or before the next recording procedure, so as to erase radiation energy of the previous radiation image still remaining in the storage panel and, in addition, to erase radiation energy given by radioisotopes inadvertently incorporated into the panel or by environmental radiation. In practice, it is required that the previously recorded image had not to appear when the next radiation image read-out procedure is performed. From this point of view, it is desired that a storage panel be excellent in the erasing property (that is, the radiation energy remaining therein be readily removable using an erasing light of relatively small energy).
A reading apparatus for reading a radiation image out of the storage panel is generally equipped with erasing means for the erasing step. In order to shorten the period of time for erasing, to reduce the weight and size of the apparatus and to reduce the operation cost, the amount of the erasing light (illuminance×time) preferably is as small as possible.